1. Field of the Invention
The present invention relates mainly to the fabrication of thin layers of solid oxygen ion conductors for high temperature electrochemical devices such as solid oxide fuel cells, oxygen gas separators, solid oxide electrolyzers for water vapor and/or carbon dioxide, and gas sensors. In the most important application the process relates to the fabrication of thin layers of oxygen ion conductors such as, stabilized zirconia, doped hafnia, and doped ceria as well as mixed conducting oxides which are conducting via oxygen ions as well as by electrons or holes. Devices which operate on the principal of oxygen ion conduction are important in the field of electric power generation, combustion control, oxygen gas separation from air, sythetic gas generation, and life support systems. It is of great importance that devices which are based on solid oxygen ion conductors and which are applied in these fields are efficient, reliable, reproducible in manufacturing, and cost competitive. The solid oxide layers are used in electrochemical cells as electrolytes with applied electrodes, or as semipermeable membranes for oxygen; they are made thin and operate at high temperatures in order to reduce electrical resistance losses.
Electrochemical vapor deposition (EVD) is a high temperature process where metal halide vapors react with oxygen in an electrochemical way, and where the desired reaction product is a thin and pore free oxide layer, deposited on top of a suitable substrate. EVD is used mainly for manufacturing of thin oxide layers which conduct electrically at elevated temperatures by the movement of oxygen ions. Mixed conducting oxides, which conduct by oxygen ions as well as by electronic charges, can also be made into thin layers by EVD, they are called here mixed conducting, oxygen-ionic/electronic, oxide layers; they are used in oxygen gas separation devices and for the generation of sythetic gas consisting of hydrogen and/or carbon monoxide.
EVD is the major process used in the manufacturing of oxygen ion conducting electrolyte layers for solid oxide fuel cell generators, when one counts the tested kilowatts as well as the delivered kilowatt hours of installed fuel cell generators as a measure for the application of the process. The most common solid oxide electrolyte, made by EVD in the form of approximately 0.002" thick layers, is yttria-stabilized zirconia. Beside EVD, other processes such as sintering, plasma spraying, physical vapor deposition, tape casting, and others, are being considered as potential processes for the manufacturing of thin layers of solid oxide electrolytes. The three most common oxygen ion conductors considered in this field of application are based on doped stabilized zirconia, doped ceria and doped bismuth oxide.
Electrochemical cells, similar in construction to fuel cells, can be used to decompose water vapor and carbon dioxide electrolytically into oxygen, hydrogen, and carbon monoxide, respectively. In such an application the oxygen ion conducting electrolyte layers, such as yttria-stabilized zirconia, must be thin and gas impervious for energy efficiency in operation which is achieved reliably by producing the electrolyte by EVD. Layers with a thickness of several micrometers can be deposited pore free and vacuum tight into porous substrates, whereby the substrate serves as structural support as well as electrical contact to one side the electrolyte.
Mixed conducting, oxygen-ionic/electronic, electrolyte layers can also be made by EVD. Such oxide materials are permeable to oxygen by ionic conduction, and gas separation devices can be constructed which separate oxygen from air by applying an oxygen partial pressure gradient across a membrane made of a mixed conducting oxide. Again, a thin layer of the mixed conductor is the key for energy efficient device operation.
Another field of application of the invention is in the manufacturing of thin solid electrolyte layers for gas sensors in combustion control application, a reduction in oxide layer thickness allows device miniaturization and cost reduction. EVD is an ideal process to manufacture such devices.
All the devices described above must be heated to operating temperatures of between 300.degree. C. and 1000.degree. C. to reduce their electrical resistance and to perform reliably. Producing thin layers of oxygen ion conductors for solid state electrochemical devices, therefore, is the field of this invention.
State-of-the-art EVD is executed at high temperatures as high as 1300.degree. C. and at low pressure which requires the operation and maintenance of vacuum pumping systems. Also, high temperature vacuum furnaces are required for the deposition of thin layer solid electrolytes. The requirement of vacuum pumps, vacuum furnaces, and their maintenance present major engineering challenges in developing the EVD process into a low cost manufacturing process. The invention relates to these engineering and manufacturing problems.
2. Description of the Prior Art
The process of EVD was first published in the Proceedings of ECS--Symposium, Electrode Materials and Processes for Energy Conversion and Storage, 1977, Vol. 77-6, pp 572-583 (A. O. Isenberg). The publication describes the basic elements of a vacuum EVD process for making thin layers of oxygen ion conducting complex oxides such as yttria-stabilized zirconia and gadolinia doped ceria, and of the mixed conducting complex oxide lanthanum chromite.
U.S. Pat. No. 4,609,562 (A. O. Isenberg, G. E. Zymboly) teaches an apparatus and method to execute the vacuum EVD process for producing yttria-stabilized zirconia electrolyte layers and layers of lanthanum chromite doped with magnesium over porous surfaces. U.S. Pat. No. 4,597,170 (A. O. Isenberg) teaches the application of the vacuum EVD process as a means to bond a porous nickel electrode to a layer of yttria-stabilized solid electrolyte by partially embedding the nickel particles into the same electrolyte material. Another patent, U.S. Pat. No. 5,106,654 (A. O. Isenberg), teaches the application of the vacuum EVD process in the formation of a composite material layer consisting of particles of chromites and manganites together with stabilized zirconia to provide a mixed conducting membrane which conducts electronically via the embedded chromite or manganite particles, and which conducts oxygen ions via the dense stabilized zirconia which surrounds the particles.
The process of EVD has been investigated with respect to growth mechanism and kinetics, as reported in the scientific literature. The following references present a good understanding of the process High Temp Sci. 27, 2512, 1990 (U. B. Pal et al.); J. Electrochem. Soc. 137, 2937, 1986 (U. B. Pal et al.; J. Electrochem. Soc. 133, 1583, 1986 (J. H. Enloe et al.). Further experimental investigations of the EVD process are reported by: J. Electrochem. Soc., 142, 3851, 1995 (H. W. Brinkman et al.) which describes the deposition of doped zirconia over porous media of different chemical composition and pore diameters. The publication J. Electrochem. Soc. 144, 1362, 1997, (R. Ioroi et al.) reports the successful EVD deposition of a double layer of two distinctly different compositions, where one layer consists of mixed conducting (oxygen-ionic/electronic) doped lanthanum manganite over a first layer of yttria-stabilized zirconia.
Most of the investigations report EVD process parameters for temperature and pressure in the range of 900.degree. C. to 1300.degree. C. and less than 1 Torr to 10 Torr, respectively. The high temperature is a requirement for achieving favorable EVD growth conditions for the thin oxide material. The low pressure in an EVD reactor beneficially affects the growth rate and quality of the deposits because the gaseous byproduct of the deposition reaction, namely halogen (mostly chlorine from vaporized metal chlorides) is removed from the reaction zone by continuous mechanical pumping. The low pressure EVD processes which are reported and described in the open patent and scientific literature reveal that only halide vapors of such metals are introduced into the reaction zone which are intended to be incorporated into the deposits as oxides. The reaction zone is identical with deposition chamber and is called here the reaction zone/deposition chamber.
This invention teaches that favorable EVD process conditions have been discovered for the fabrication of oxygen ion conducting and mixed conducting oxide layers on porous support structures at or near atmospheric pressure which has not been accomplished as of to date. This invention, therefore, represents a major cost reduction step in the manufacturing of thin and pore free oxygen ions conducting layers for high temperature electrochemical devices, such as oxygen generators, solid oxide fuel cells, solid oxide electroysis cells, gas separator devices, and combustion and process control sensors, all of which require that the thin oxide layers are deposited onto porous electrodes or onto other porous support structures to insure mechanical integrity of the devices.